We are studying the early stages of peripheral neuronal differentiation by grafting quail neural crest or its ganglionic derivatives into young chick embryo hosts and following the subsequent migration and differentiation of the implanted cells by cytological and cytochemical techniques. In this way, we aim to determine the stage(s) at which specific neuronal phenotypic programmes (e.g. sensory, autonomic, cholinergic, adrenergic) are acquired. Combined 3H-thymidine autoradiography and catecholamine fluorescence cytochemistry should enable us to determine whether the developmental state of post-mitotic neurons can be modified experimentally under in vivo conditions. In parallel to these experiments, we are investigating the factors involved in the morphological and chemical differentiation of neurons in cultures of microsurgically excised neural crest cells, alone and in association with young embryonic tissues of non-neural crest origin. In particular, we hope to simplify the system by eliminating serum from the culture medium. We attach special interest to the existence of early cholinergic properties in neural crest and experiments will be performed to attempt to determine their significance.